How much were you planning on spending per child?
If it's $10 and under, Target (before Christmas) had board games for under $10. DD likes the game Trouble.
For the boy, you could get a Nerf football, all boys like Nerfs.
Or you could get them gift cards to the movies, Target, McD., Chic Fil A.
I usually base the price on how close my kids are to the person who is having a party.
If it's $15-20, I usually get a $10 gift card to the movies, then get a Popcorn movie bag (or Target had plastic Popcorn containers in the $1 spot), I fill it with movie type candy and put the gift card in it
OR
I get a $10 GC from Blockbuster, buy the unpopped popcorn bucket from there, put some candy, a Sprite and the GC in it.
Girls also like little nail kits, you can usually get those at
Walmart or Target for $5-10.
I think Target sales Build a Bear outfits, you could get a couple of those for the B-A-B parties.
Hope this helps.